1. Field of the Invention
The present invention relates to a servo control system used in an optical disk drive unit to correctly focus illuminating light onto the data recording surface of an optical disk. More specifically, the present invention relates to a focus servo control system provided in an optical disk drive unit and having a track servo loop for positioning an optical head at a predetermined point above the desired track on an optical disk and a focus servo loop for controlling the focus of a spot of light applied to the optical disk from a laser light source provided in the optical head. The focus servo control system controls the focus servo loop on the basis of the "on/off" state of the track servo loop.
2. Description of the Related Art
Recently, various digital data recorders have been developed actively. Among them is the optical disk drive unit. Since it enables writing and/or erasing of data at high density and also allows transfer of data at high rate, the optical disk drive unit is recognized as the most attractive device.
FIG. 1 is a block diagram showing the arrangement of a typical optical disk drive unit, which is denoted by reference numeral 33. The optical disk drive unit 33 is connected to and controlled by a host computer 34. The optical disk drive unit 33 has some controllers, for example, an upper-level controller 6, a controller 3A, a servo controller 3B, a light-volume controller 35, and a spindle controller 37.
Reference numeral 1 denotes an optical disk, which is driven by a spindle motor 36 that is controlled by the spindle controller 37. Further, reference numeral 38 denotes a magnetic field generator, which enables data to be written to and read from the optical disk 1 by an optical head, which is denoted by reference numeral 2. The optical head 2 is moved radially of the optical disk 1 by a voice coil motor (not shown) and positioned to a sought track.
The upper-level controller 6 serves as an interface between the optical disk drive unit 33 and the host computer 34. In response to control commands from the host computer 34, the upper-level controller 6 sends data to be written to a write-circuit 4 and receives read-data from a read-circuit 5.
In response to a write instruction from the upper-level controller 6, the write-circuit 4 transfers data to be written to the optical head 2 and controls the power of light emitted from a light source (laser diode) provided in the optical head 2 during writing.
In response to a read instruction, the read-circuit 5 binalizes data read by the optical head 2 and transfers a clock signal and the read-data, which is synchronized therewith, to the upper-level controller 6.
In this optical disk unit 33, the servo controller 3B constitutes servo loops that control the optical head 2 under control of the controller 3A. The servo loops include a track servo loop and a focus servo loop.
The track servo loop executes control so as to move the optical head 2 to a predetermined position above the desired track on the basis of a seek instruction from the upper-level controller 6 or the signal read by the optical head 2 under control of the controller 3A.
On the other hand, the focus servo loop controls the focus of a spot of light from the laser light source on the basis of the signal read.
More specifically, the optical head 2 is moved radially of the optical disk 1 so as to be positioned above the desired track under control of the track servo loop. In addition, in order to control the focus of the spot of light from the laser light source, the optical head 2 is moved perpendicularly to the surface of the optical disk 1 under control of the focus servo loop.
In the above-described track and focus servo loops, servo error signals (i.e., a track error signal TES and a focus error signal FES, respectively) are generated in accordance with differences between the target and present positions of the optical head 2.
These servo error signals are passed through respective amplifiers and led to respective optical head movement control circuits, that is, a track actuator coil TC for positioning the optical head 2 to the desired track, and a focus actuator coil FC for moving the optical head 2 perpendicularly to the optical disk 1 to control the focus of the spot of light from the laser light source.
Incidentally, since the optical disk 1 has tracks spirally formed thereon, the track servo loop needs to execute kickback control for each full turn of the optical disk 1 in order to maintain the optical head 2 above the same track. Each time the optical head 2 crosses one track, a peculiar sine waveform appears in the track error signal TES.
In the focus error signal FES in the focus servo loop also, a peculiar waveform appears in correspondence to the sine waveform due to crosstalk, although this peculiar waveform is minute.
In a normal state, the track servo loop is in an "on" state except when it is turned off in response to a command from the controller 6. However, when it is difficult to perform normal tracking due to, for example, a flaw in the surface of the optical disk 1, the track servo system is turned off. In such a case, the waveforms of the track and focus error signals TES and FES become disordered. In particular, when the optical disk 1 has a large eccentricity, the frequency, that is, the number of times, at which the optical head 2 crosses a track increases. Consequently, there is an increase in the current that is passed through the focus actuator coil FC of the optical head 2 for the focus control.
If this state continues long, the temperature of the focus actuator coil FC rises, causing the coil FC to be deteriorated, resulting in a decrease in the lifetime of the coil FC.